Did you really intend to take the 300th derivative? I get the impression you don't really understand what the diff function is doing.
When you put in an array, if will calculate the difference between subsequent values. So it doesn't really find the derivative, as it find the numerical approximation of the derivative on the midpoints.
x = [1 2 3];
y = [7 3 6];
diff(y),corresponding_x=mean([x(1:(end-1)) ; x(2:end)])
